Hematopoiesis and hemocyte reservoirs control the number of circulating hemocytes in the apple snail Pomacea canaliculata (Mollusca, Gastropoda)

Pomacea canaliculata is an invasive freshwater snail native to South America and introduced in Southeast Asia, where it severely damages rice seedlings and acts as intermediate host of the parasite Angiostrongylus cantonensis. After its retrieval in Spain, the European Union Parliament banned P. canaliculata import and diffusion. Information about the immune system of P. canaliculata is needed to implement the control measures against its diffusion. The principal cellular players of molluscan immune system are the circulating hemocytes. P. canaliculata hemocytes consist of blast-like cells (20%), hyalinocytes (72%) and granulocytes (8%). All these cells adhere to glass surface, but only hyalinocytes and granulocytes phagocytize heat-inactivated bacteria. Four hemolymph withdrawals in 72 h did not change the hemocyte total number but blast-like cells slightly decreased. The p(Ser10)-H3 mitotic marker was evidenced in cells along the external side of the vessels entering the heart. These observations were indicative of an active hematopoietic tissue, localized into the pericardial cavity. The repeated withdrawals had no effects on hemocyte renewal rate, thus one or more hemocyte reservoirs may intervene in buffering hemocyte lost in P. canaliculata. Light microscopy observations suggested that the ampulla acts as hemocyte reservoir. These results demonstrate that adult P. canaliculata possess an active hematopoietic district like Biomphalaria glabrata and other gastropods, whose amebocyte-producing organ (APO) consists of clusters of cells positioned between the pericardial and the posterior mantle epithelia. Further, hematopoiesis role in hemocyte renewal is supported by the ampulla, that seems to be a hemocyte reservoir in P. canaliculata.